This invention relates to a frequency attenuator or converter, i.e. divider, with an infinitely variable reduction ratio.
Such frequency converters may, for example, be used in tachometers and tachographs, i.e., with apparatuses which record or indicate a rotational or linear speed, or measure a distance covered, or any combination of these.
When speed is measured electronically, a suitable pulse generator must transmit at least a sufficient number of pulses per unit distance, using a reasonable number of circuit elements, so that a steady speed indication exists even when the speed is low. However, these high frequencies required for speed measurement are not necessary for measuring distances. For that, a pulse rate of one pulse per 100 meters is adequate. In fact, the high frequencies suitable for speed measurements are incompatible with the need of distance measuring devices using distance indicators or other readout devices using a stepping motor or other electromechanical driving device. That is to say, a motor or electromechanical driver may not be able to respond to pulses at high repetition rates. The frequency of the pulses must either be reduced or special pulse generators must be used.
For purposes of production as well as inventory organization, known indicating or recording apparatuses of this type are usually adapted for operation in response to definite number of input pulses per revolution or a definite number of pulses per unit distance. This number of pulses per revolution or pulses per unit distance can be considered an input constant. However, vehicles do not often deliver pulse rates that correspond to such an input constant. Vehicles and vehicle connections vary. Depending on the type of vehicle, different numbers of revolutions per unit distance may be sensed at difference places between the vehicle wheel and the motor. The position at which sensing takes place is determined by the vehicle manufacturer. Moreover, the same type of vehicle may operate with various sizes of tires, or the vehicle maker may utilize pulse generators having varying numbers of pulses per revolution.
It is possible to accommodate these variations in pulse rates by providing a specific frequency divider, for each particular situation. However, this is not very advisable. Rather, each tachograph or tachometer should have associated therewith a frequency transducer with an infinitely variable (continuously variable) reduction ratio over a specific frequency range. In this way a tachograph or tachometer can operate with the pulse frequencies produced by virtually any pulse generator of any vehicle.
Of course, it is also possible to obtain a substantially exact adaptation of pulse rates on a stepwise basis by changing gears in a reduction gear arrangement inserted between the drive shaft and the indicating rollers of a device such as a distance indicator in a tachograph. It is also possible to adjust for different pulse rates by changing gears in a gear arrangement that drives the pulse generator. This type of structure is usual when the revolution count is transmitted by mechanical driving means.
However, when processing data electronically, such mechanical devices are highly unsatisfactory. This is so not only because of the transmission errors inherent in such mechanical systems, but also because of the expense of production, mounting, and storing, the large number of gear combinations. Furthermore, gearing demands a great deal of effort insofar as organizing the arrangement is concerned.
According to one known circuit of arrangement, programmable electronic switching means effect the desired frequency adjustment by issuing a definite number of counting pulses of high frequency for each pulse generator input pulse. The number of high frequency pulses is determined by the desired reduction ratio. This is a simple manner of permitting adjustment of the pulses delivered from the vehicle driven pulse generator to the tachograph or tachometer input constant. Apart from the necessary pulse forming stage, such an arrangement requires an oscillator, and to obtain the required accuracy, a multistage counter and a multistage switching circuit, as well as a comparison circuit. It is as accurate as the usual mechanical adjusting means. It is, therefore, too expensive for normal use in view of the fact that the sum of the pulses produced for each generator input pulse must be reduced decadically at a later stage to permit further processing.
Another arrangement of this kind attempts to use less expensive electronic circuitry. However, it still requires an oscillator which produces counting pulses of higher frequency than the generator frequency. However, the output frequency must again be reduced to achieve a sensible operating frequency. Here the reduction ratio is defined by a monostable multivibrator producing pulses of an adjustable pulse width in response to the control of the generator pulses. This circuitry permits high accuracy only at relatively high counting frequencies and at great expense which results from the frequency reduction means. Apart from the above, the accuracy of the circuit is dependent upon the oscillator maintaining a constant frequency and avoiding the influence of temperatures upon the monostable multivibrator. Additional difficulties might arise from extraneous disturbances becasue this system must be used in a vehicle.